feat: support extending JSONFormatter

[atsjo]

Support extending JSONFormatter with custom text diff operations, and fix a potential bug where MovedDelta is identified as a TextDiffDelta:

 if (itemDelta[2] === 3) { throw new Error( "JSONPatch (RFC 6902) doesn't support text diffs, disable textDiff option", ); } 

As of now I have to make my own copy of JSONFormatter to extend it. By accepting this PR this would be a cleaner integration for me, and give other the same easy way of supporting custom text diffing operations by extending JSONFormatter like this:

class MyJSONFormatter extends JSONFormater { protected override processTextDiff(ops: Op[], path: string, diff: string) { const lines = parseTextDiff(diff); for (const line of lines) ... } }